This invention relates to antenna rotation, and more particularly to a control circuit using antenna position feedback for driving an antenna rotator.
Radio and television receivers employ antennas for the reception of the broadcast electromagnetic radio and television signals. For home viewing of television programs, as well as listening to FM (frequency modulation) radio programs, the antenna may be placed in the room alongside the receiver, or alternatively, may be mounted on the roof. Roof mounting is preferred for the reception of distant signals since the higher elevation of the antenna increases the strength of the signal which is received.
For further improvement in signal reception, the antenna may be designed for high directivity, as with an array antenna, and oriented so as to point directly at the source of the television (or radio) broadcast. In order to change the orientation of an antenna so as to receive broadcasts emanating from different sites, the antenna is typically mounted on a rotator which, in turn, is secured to the roof. Normally, the rotator is electrically actuated by a control circuit which includes a control panel adjacent to the receiver for allowing the viewer to point the antenna in the desired direction. A person viewing a television program can readily select the desired direction of the antenna by turning a knob on the control panel.
The design of such electrical circuits for the control of antenna rotators is subject to various constraints which create numerous problems, and which impede the design of a circuit which can be commercially acceptable while providing satisfactory operation.
One problem attending the design of an acceptable control circuit is the choosing of a motor to rotate the antenna, i.e., whether an AC (alternating current) motor or a DC (direct current) motor is to be utilized. While a DC motor (which has a permanent magnet rotor) of a given size can provide more torque than an AC motor of comparable size, the brushes of a DC motor produce electrical disturbances (called electrical noise) which are relatively strong when compared to the strength of the electric signals appearing in the solid state circuitry used in televisions and radios (e.g., transistors and integrated microcircuits). As a result, the noise of the brushes may well be strong enough to induce distortion of a television picture or radio broadcast whenever the motor is in operation for rotating the antenna, making it impossible for one to hear the radio broadcast or view the clarity of the television picture being received while simultaneously turning the antenna. Accordingly, it has been the practice to use AC motors which do not have brushes but are, unfortunately, of substantially larger size, larger weight, and more costly.
Further design problems arise in the area of the electrical connection between the antenna rotator drive circuit located at the receiver, and the components of the system located on the roof, adjacent the antenna.
For example, the number of conductors which can be run between a drive circuit and any system components located on the roof is limited to five under a prevailing industry standard. Thus, a multiconductor cable having only five conductors typically provides such connections. The five conductors must carry all necessary power to the motor as well as any other control signals which might be utilized by the circuit in the operation of the rotator.
In connection with this, past circuit designs have failed to use the circuit configuration which it has been found can provide the most accurate positioning of an antenna, i.e., a closed-loop control circuit employing a feedback signal indicating the orientation of the antenna. Since the use of a feedback signal would necessarily require additional electrical conductors in the connecting cable, in addition to those conductors utilized for simply powering the motor, it has been common practice to utilize open-loop control circuitry for positioning antennas. Thus, the industry standard requiring the use of cables having five wires or less has heretofore imited the selection of available circuit configurations.
An additional design problem becomes evident in the implementation of circuits using a multiconductor cable to power a motor and to conduit feedback signals indicating the orientation of an antenna. Here it has been found that a single wire conductor of the cable cannot be used as a common ground return for both the motor power and the feedback signals, since the common ground being lengthy, would serve as a medium of excessive coupling between the input and output terminals of the control circuit, thereby causing the control circuit to become destabilized.
Another problem is the need of any antenna rotator control circuit using a closed-loop drive circuit to differentiate between true feedback signals (indicating a true variation between the selected position of the antenna and its true position) and spurious feedback signals caused by vibratory movements of the antenna due to wind gusts.
Finally, other problems require the circuitry to be designed so as to preclude burn-out of the motor in the event that the antenna is jammed by ice in the winter time, or in the event that the wiring is incorrectly installed, as may occur during installation by an inexperienced person.